Heretofore, microwave/milliwave circuits which perform the amplification, oscillation, mixing, etc. of radio frequency signals, and digital circuits which perform the amplification, identification, branch, etc. of digital signals have been put into practical use in various systems.
Among the various circuits, a wideband amplifier of distributed type for use in an optical communication system or a radio communication system will be described with reference to FIG. 1. FIG. 1 is a circuit diagram showing an example of the distributed amplifier in the prior art.
The distributed amplifier in the prior art is a cascode-type distributed amplifier of three-stage configuration as employs HBT cascode pairs 3 each being constructed by cascode-connecting a hetero-junction bipolar transistor (hereinbelow, abbreviated to “HBT”) 1 and an HBT 2. An input signal is received from an input terminal 7, while an output signal is delivered from an output terminal 8. The base terminal of the HBT 1 is fed with a DC supply voltage from a base power source 9 through a terminating resistor 12. The collector terminal of the HBT 2 is fed with a DC supply voltage from a collector power source 10 through a terminating resistor 12. The base terminal of the HBT 2 is fed with a DC supply voltage from a cascode power source 11 through a terminating resistor 12. Besides, the base terminal of the HBT 2 is grounded radio-frequency-wise through an RF grounding capacitor 13.
The distributed amplifier stated above forms a transmission line of high cutoff frequency by combining the parasitic reactances of the HBTs 1 and HBTs 2 with high-impedance transmission lines 4 and 5. The transmission line has a characteristic impedance which is equal to each of a signal source impedance and a load impedance, and it can realize a flat gain and a low reflection loss over a wide band.
With the cascode-type distributed amplifier in the prior art as shown in FIG. 1, however, a reflection loss particularly on an output side increases outside a required frequency band. There has accordingly been the problem that the reflection loss leads to the occurrence of a negative resistance in some cases. There has also been the problem that the stability of the circuit is consequently degraded to give rise to a parasitic oscillation or an unstable operation.
Incidentally, although the distributed amplifier has been exemplified here, the problems explained above are problems which can occur in various circuits such as microwave/milliwave circuits which perform the amplification, oscillation, mixing, etc. of radio frequency signals, and digital circuits which perform the amplification, identification, branch, etc. of digital signals.
The present invention has been made in view of the above problems. An object of the present invention is to connect a reflection loss suppression circuit to any of various circuits including a distributed amplifier, whereby a reflection loss outside a required frequency band is satisfactorily suppressed without degrading the characteristics of the circuit in the required frequency band, so as to attain the stability of the circuit.